The present invention relates to a method for evaluating and managing a resist pattern formed on a wafer in a semiconductor manufacturing process, and more particularly to a technique for measuring and evaluating the amount of film thickness reduction of a resist (a reduction in height of a resist pattern) using an electron microscope image of the resist pattern.
Conventionally, a length-measuring scanning electron microscope (SEM) which is an electron microscope for the purpose of measuring a dimension of the pattern is widely used as a process management tool in a lithography process. The length-measuring SEM enables imaging at high magnification of a hundred thousand times to three hundred thousand times, and thus can measure the dimension of a fine pattern of the order of several tens of nanometers with an accuracy of 1 nanometer or less. The basic structure of the length-measuring SEM is disclosed in Tatsuhiko Higashiki, Ed. “Photolithography II—Measurement and Control—”, ED Research Co., Ltd., pp. 31-41 (2003).
The lithography process involves transferring a circuit pattern onto a wafer by exposure and development of a resist, and etching along the resist pattern transferred. The length-measuring SEM is used to measure the dimension of the transferred resist pattern or the pattern etched. In particular, wiring patterns including a transistor gate wiring are subjected to strict dimensional control because the width of the pattern is strongly related to device performance.
FIG. 2A shows sectional shapes of the patterns before etching, during etching, after etching in that order from left side thereof. The length-measuring SEM measures a resist pattern width W1, or a pattern width W2 after the etching.
In a conventional lithography process, the resist pattern width W1 within the standard allows the pattern width W2 after the etching to be restrained within the standard. Thus, sufficient process management is available by monitoring measurement results of the widths W1 and W2.
In order to satisfy the need for microfabrication of patterns, a high NA exposure technique has been developed for obtaining a resolution required for formation of a fine pattern. As a result, a margin for the exposure process becomes small, and a small variation of an exposure parameter, such as a dose amount or a focus position in exposure, leads to film thickness reduction of the resist pattern, that is, a decrease in height of the resist as compared to the case of the normal exposure.
As shown in FIG. 2B, when a height of a pattern is low (h1<H1) even with the same pattern width before etching as that shown in FIG. 2A (w1=W1), a pattern width after the etching becomes smaller (w2<W2). During the etching, the thickness of the resist is gradually decreased. When the original pattern thickness is low, the resist pattern almost disappears during etching, and then a film of interest for process may be itself etched as shown in the center drawing in FIG. 2B. As mentioned above, the pattern width after the etching is directly related to the device performance. The condition which may cause the reduction in pattern width after the etching as shown in FIG. 2B is not appropriate.
In order to introduce the high NA exposure technique, only measurement of the pattern width (W1, w1) is not sufficient in a resist pattern stage from a point of view of process management, and also a pattern height (H1, h1) is desired to be measured together with the width. Methods for measuring a pattern height can include measurement using an electron microscope for observation of a section of a wafer by dividing the wafer, and measurement using an atomic force microscope (AFM). The former needs dividing the wafer, which cannot be apparently applied to the normal in-line process management. The latter is not appropriate for use in monitoring the process which needs high throughput.